Light conversion layers are used wherever the spectral composition of light sources needs to be changed. In contrast to color filters, which absorb or reflect certain spectral components of light, light conversion layers convert part of the absorbed energy of light and radiate it again at a higher wavelength. It is therefore possible to convert light of low wavelength into light of higher wavelength. The conversion layers contain, for this purpose, one or more conversion dyes which absorb at least part of the light spectrum and are able to convert it by fluorescence or phosphorescence.
This method of light conversion is used for example for the creation or modification of illuminants that are visible to the human eye (light wavelengths between 400 nm and 700 nm). Such illuminants are used wherever light is needed which is visible to the human eye, for example for the illumination of rooms, the illumination of advertisements and security markers, or for the background lighting of LCD screens and mobile phones.
A well-known example of such an illuminant is the fluorescence tube which generates ultraviolet radiation that is invisible to the human eye and which is converted by a light conversion layer into light having a wavelength between 400 nm and 700 nm. The light conversion layer is applied directly to the inner surface of the fluorescence tube and only inorganic phosphors are used as conversion dyes. A further property of these layers is that they need to be impermeable to ultraviolet light and strongly scatter visible light. Therefore, they are not transparent, but opaque.
A further example is a white light emitting diode wherein, for example, blue light is converted in part into yellow-green light by a conversion dye, as described for example in patent application WO 2006/127,030.
Light conversion layers are also used in security markers wherein, for example, ultraviolet light that is invisible to the human eye is converted into visible light. Such a security marker is only visible under an ultraviolet lamp.
Light conversion layers are also used in food production, wherein, for example, the light conversion layers convert sunlight or artificial light in such a way that plant growth is accelerated.
Light conversion layers are also used to change the light spectrum of self-luminescent phosphorescent areas in the dark, as used for example for signaling emergency exits.
All conversion layers have the same objective of efficient conversion of light and of radiation of converted light in a wanted direction. For this purpose, the excitation of the conversion dyes by the light source and the light conversion have to be very efficient. The excited conversion dyes are internal light sources inside the conversion layer. The converted light then needs to be extracted from the layer. The ratio of emitted light quantity and of absorbed light quantity is called external fluorescence efficiency. In order to optimize the function of the conversion layers, it would be an advantage it the individual steps (excitation, conversion, extraction) could be optimized independently as far as possible.
Inorganic phosphors having particle sizes in the micrometer range, embedded in a transparent polymer, are used in widely-used conversion layers, as described for example in patent applications JP 11-199,781 or US 2007/0,031,685.
Due to the difference of the refraction index between the particles and the polymer, and particle size, as described in these documents, the conversion layers prepared in this way are opaque and considerably scatter visible light. This considerably hinders or even prevents separate control of absorption and emission by the conversion dyes. Although the used phosphors have often very high quantum yields, the external efficiency of light conversion by the conversion layers is considerably deteriorated, because a large amount of the converted light cannot escape from the conversion layer or is radiated in an unwanted direction.
There is a further problem in the case where more than one conversion dye is used. The propagation behavior of the light in the conversion layer depends on the used wavelength. Therefore, the optical properties of the conversion layers should be adjusted to the used conversion dye. It is also necessary that the absorption of the excitation light by the different conversion dyes and the reciprocal filter effect on the emitted radiation should be carefully controlled.
In order to attain this objective, multi-layer systems containing the different conversion dyes have been developed, wherein the optical properties of the obtained conversion film may be controlled selectively.
The multi-layer systems, described for example in patent applications EP 1,480,278 or WO 2006/114,077, allow only a very limited control of optical properties due to strong scattering. In particular, only the sequence of the conversion layers containing the two conversion dyes is used to increase conversion efficiency. However, the scattering behavior and light coupling out of light may be controlled only marginally.
By using organic dyes in place of the inorganic phosphors mentioned above, transparent conversion layers having only moderate scattering of light may be prepared. Such a layer is described in patent application EP 1,422,282.
Beside the use of organic fluorescence dyes, the use of inorganic phosphors in combination with polymers having a high refraction index, such as, for example, polyimide, for the preparation of nearly transparent layers is described in patent application WO 2005/100,016. The required transparency is obtained by the adjustment of the refraction indices of the polymer and the conversion dye. Although nearly transparent layers may be prepared in this way, the distribution of the conversion dyes in the layers is defined by particle size. The layer consists therefore of zones of micrometer size having incorporated a conversion dye and equally big zones of inactive transparent polymer, leading to a very inhomogeneous absorption and emission behavior of such layers.
Patent application WO 2005/051,846 describes the preparation of inorganic phosphor particles having a size between 20 nm and 800 nm. Such particles would be very suitable for the preparation of transparent conversion layers, because the particles are small in relation to the wavelength of the used light and scattering would therefore be lowered considerably.
However, if such transparent conversion layers are used, the problem appears that the converted light in multi-layers is captured by shafting and is mainly radiated in a sidewise direction. This is undesirable in the applications described above.
The partial suppression of radiation in a sidewise direction by shafting by the use of a microporous and mesoporous layer having a low refraction index is described in patent application EP 1,492,389.
In patent application WO 2005/100,016, the use of multi-layers also is described, wherein, based on transparent conversion films, coupling out of light is purposefully controlled by the use of supplementary optical elements. The materials and preparation processes described therein are not suitable for cost-efficient and large-scale production of such conversion films.